RADIOLOGY

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or blackening, a feature which is common to almost all metals when functioning as a negative electrode in a vacuum. Aluminium shows the effect but slightly, but with very heavy discharges the central portion of the cathode may be melted, the molten globules so formed being projected right across the tube in some cases. No particular harm is usually done, except that the curvature of the cathode may be changed and the focal spot of the cathode rays moved in consequence.

The choice of metal for cathode is also important from the point of view of the occluded gas. There is little doubt that in the case of a gas tube the gases contained within the metal of the cathode play an important part. Tliis is probably the explanation why some experimenters have found it useful to keep the cathode cool by some device.

The cathode of a Coolidge tube is described later.

§ (10) THE ANTiOATHODE.-^-The primary essentials in an anticathode, whether for gas or hot-cathode tube, are :

1. A high atomic number to secure a large output of X-rays.

2. A high melting-point and a high thermal capacity to prevent the target fusing under a heavy discharge.

3. A high thermal conductivity to assist in dissipating the waste heat.

4. A low vapour pressure to avoid distillation and condensation of the motal on the walls of the tube.

In the light of the above requirements, tungsten stands almost alone as a material for anticathodes. Platinum was at one time almost exclusively employed, but tungsten has a much higher melting-point (3200° C. as against 1755° C.), has twice the thermal conductivity, and but a slightly inferior radiation value.

The very large heating effects experienced by the anticathode of a modern X-ray bulb have provided scope for much ingenuity in getting rid of the surplus heat. The anti-cathode is in many tubes kept cool by means of a water reservoir or a stream of air. In other oases the massiveness of the anticathodo is increased by surrounding the tungsten . target with copper, the support of which extends to the outside of the tube and is there provided with radiating fins, incidentally necessitating some very fine glass manipulation.

§ (11) THE ANODE.—The modern X-ray gas bulb is almost always provided with an additional anode of aluminium, which is connected externally with the antioathodo. The precise benefits of the separate anode are distinctly doubtful, and in tubes of the Coolidge type it finds no place.

§ (12) THE GAS TUBE.—The gas tube depends for its action on the presence of a few ions in the residual gas in the tube. These ions or electrified atoms have their velocities increased by the electric field, positive ions being drawn to the cathode and negative to the anode. The positive ions bombarding the cathode release electrons in abundance which, being attracted to the anode, ionise freely by shock or collision those atoms encountered en route, generating more positive ions and more electrons. The electrons which hit the target generate X-rays and the cycle of operations continues so long as the voltage is applied.

The positive ions or positive atoms thus play a fundamental and essential part in the ionics of a gas tube. They are also responsible for one or two other effects, tho elucidation of which has boon vory puzzling. One of tho great difficulties in exact work with the gas tube is the continual tendency of tho gas pressure to change. One would first look to tho electrodes which, depending on tho conditions, may either omit or absorb gas and do so control very materially the well-known " crankiness " of a gas tube. But it is found that, provided tho current is not too heavy to overheat the electrodes, there is a continual and apparently unlimited disappearance of gas, more especially at high voltages, and ultimately tho vacuum becomes so high as to render tho tube unusable. To cut a long story short wo now know that some of tho positively charged atoms of gas by reason of their high velocity (about 500 miles a second) actually crash into tho glass walls of tho tube and are mechanically trapped thoro, an effect which is enhanced by tho presence of volatilised motal.

Many devices have boon introduced from time to time to overcome tlua hardening effect. In some cases, when the tube becomes too hard, tho discharge is caused to pass through a small annexe containing absorbent material, such as asbestos, which liberates enough gas to soften tho tube. Another method commonly employed is to rely on tho diffusion of gas through a small platinum tube, which can be heated by a small flame.

If a gas tube is overloaded tho result is sometimes to harden tho tube, sometimes to soften it, depending largely on tho behaviour of the cathode. Further, if the discharge is sufficient to make tho target rod-hot, it may then give off so much gas as to necessitate ro-oxhaustion of the bulb.

A very commonly met source of failure of gas tubes is due to cracking of the glass, almost always in the region round the cathode. In many cases this is preceded by a roughening of tho glass at this point. Those effects are produced by positive rays striking tho glass walls. The positive rays are probably also responsible for tho melting of tho aluminium

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